“Terrorists have routinely bombed public transport and railway stations, in particular,” Peter Powel, a crisis management specialist from London told RT.

Powell cites the history of such attacks worldwide, including the 7/7 attacks on London Tube.

At that time 5 years ago, 56 people were killed and up to 700 injured when three bombs exploded on the underground and one more on a double-decker bus.

“We all have to use trains and aircraft frequently. So the impact is apparent, straight away and made more so, as we saw this morning in Moscow. Within seconds, minutes on Twitter, on Youtube all these social networks, the images were at once and apparent,” Powell added.

More than that, Powel speaks of the so-called “legacy” issue. The impact of these terrorist attacks can be felt months and years later when people are constantly reminded of the tragedies by having to undergo all the extra security checks at airports and railway stations.

“Every time we have to queue up for a train or queue up for an aircraft especially, we are reminded of the terrorists,” says Powell.

According to the specialist, ideally every passenger should be the “eyes and ears of the security services”, always alert to report potential terrorist to the police.

“But that’s easier said than done,” he admits, saying that one of the biggest issues in the way is that it is difficult to say exactly what terrorists look like these days.

“It is extremely difficult to prevent any kind of terrorist attack on large scale public transport systems such as the Metro in Moscow, the London Underground, and even the metro and transport systems around Madrid, as we have seen,” Clements told RT. “The trouble being that so many people use it on a daily basis, it becomes extremely difficult to carry out any kind of systematic security search on these people, especially as they are often entering from so many different points of the system.”

Paul Wilkinson, Emeritus Professor of International Relations, thinks the threat of more attacks across the globe is very real, “because we face a form of transnational terrorism from the global Jihadi movements, some of them under the Al-Qaeda banner, some affiliated in a loose way and some borrowing the ideology of Al-Qaeda.”

“And they are all dedicated to mass casualty attacks – the typical weapon is suicide bombing in a coordinated attack and of course they are looking for public places, public transport systems, aviation and other similar targets where they know there will be large numbers of civilians gathered,” he added.